Health effects of tea
According to legend, the health effects of tea have been examined ever since the first infusions of Camellia sinensis about 4700 years ago in China. Emperor Shennong claimed in The Divine Farmer's Herb-Root Classic that Camellia sinensis infusions were useful for treating a variety of disease conditions.
Historically as well as today, in regions without access to safe drinking water, the boiling of water to make tea has been effective in reducing waterborne diseases by destroying pathogenic microorganisms. Recently, concerns have been raised about the traditional method of over-boiling tea to produce a decoction, which may increase the amount of pesticides and other harmful contaminants released and consumed.
Aluminum and heavy metals
Tea drinking accounts for a high proportion of aluminum in the human diet. The levels are safe, but there has been some concern that aluminum traces may be associated with Alzheimer's disease. A recent study additionally indicated that some teas contained possibly risky amounts of lead (mostly Chinese) and aluminum (Indian/Sri Lanka blends, China). There is still insufficient evidence to draw firm conclusions on this subject.
The US National Cancer Institute reports that in epidemiological studies and the few clinical trials of tea for the prevention of cancer, the results have been inconclusive. The institute "does not recommend for or against the use of tea to reduce the risk of any type of cancer." ... "Inconsistencies in study findings regarding tea and cancer risk may be due to variability in tea preparation, tea consumption, the bioavailability of tea compounds (the amounts that can be absorbed by the body), lifestyle differences, and individual genetic differences." Though there is some positive evidence for risk reduction of breast, prostate, ovarian and endometrial cancers with green tea, it is weak and inconclusive.
Meta-analyses of observational studies have concluded that black tea consumption does not appear to protect against the development of oral cancers in Asian or Caucasian populations, the development of esophageal cancer or prostate cancer in Asian populations, or the development of lung cancer.
A 2013 Cochrane review of randomized controlled trials (RCT) greater than 3 months duration concluded that long-term consumption of black tea slightly lowers systolic and diastolic blood pressures (about 1-2 mmHg). This conclusion was based on limited evidence. Another meta-analysis of RCTs reached a similar conclusion.
All tea leaves contain fluoride; however, mature leaves contain as much as 10 to 20 times the fluoride levels of young leaves from the same plant. Although low concentrations of fluoride are maintained in many public water supplies for dental health, very high fluoride intake (over 2 mg per day for children, 4 mg adults) increases the risk of osteofluorosis and fractures. There is evidence that over-intake of teas produced using mature leaves (e.g. brick tea) can cause fluorosis in humans.
The fluoride content of made tea depends on the picking method and fluoride content of the soil in which it is grown; tea plants absorb this element at a greater rate than other plants. Care in the choice of the location where the plant is grown may reduce the risk. It is speculated that hand-picked tea would contain less fluoride than machine-harvested tea, because there is a much lower chance of harvesting older leaves during the harvest process. A 2013 British study of 38 teas found that cheaper UK supermarket tea blends had the highest levels of fluoride with about 580 mg per kilogram, green teas averaged about 397 mg per kg and pure blends about 132 mg per kg. The researchers suggested that economy teas may use older leaves which contain more fluoride. They calculated a person drinking a litre of economy tea per day would consume about 4 mg of fluoride, the maximum recommended amount of fluoride per day but below the maximum tolerable amount of 10 mg fluoride per day.
Tea contains oxalate, overconsumption of which can cause kidney stones, as well as binding with free calcium in the body. The bioavailability of oxalate from tea is low, thus a possible negative effect requires a large intake of tea.
Theanine and caffeine
Tea also contains theanine and the stimulant caffeine at about 3% of its dry weight, translating to between 30 mg and 90 mg per 8 oz (250 ml) cup depending on type, brand and brewing method. Tea also contains small amounts of theobromine and theophylline. Due in part to modern-day environmental pollution fluoride and aluminium have also been found to occur in tea, with certain types of brick tea made from old leaves and stems having the highest levels. This occurs due to the tea plant's high sensitivity to and absorption of environmental pollutants. Dry tea has more caffeine by weight than dry coffee; nevertheless, more dry coffee than dry tea is used in typical drink preparations—which results in a cup of brewed tea containing significantly less caffeine than a cup of coffee of the same size.
The caffeine in tea is a mild diuretic. However, the British Dietetic Association has suggested that tea can be used to supplement normal water consumption, and that "the style of tea and coffee and the amounts we drink in the UK are unlikely to have a negative effect [on hydration]".
Green tea consumption has no meaningful effect in aiding weight loss.
- N. H. Woodward, Teas of the World (1980), as cited in D. A. Balentine, M. E. Harbowy, H. N. Graham, Tea: The Plant and Its Manufacture; Chemistry and Consumption of the Beverage in Caffeine ed G. Spiller (1998)
- Abd El-Aty AM, Choi JH, Rahman MM, Kim SW, Tosun A, Shim JH (2014). "Residues and contaminants in tea and tea infusions: a review". Food Addit Contam Part A Chem Anal Control Expo Risk Assess 23 (September): 1–11. doi:10.1080/19440049.2014.958575. PMID 25164107.
- "Black tea". MedlinePlus, US National Library of Medicine. 2015. Retrieved 15 March 2015.
- Streeta R, Drábeka O, Szákováb J, Mládkováa L (2007). "Total content and speciation of aluminium in tea leaves and tea infusions". Food Chemistry 104 (4): 1662–1669. doi:10.1016/j.foodchem.2007.03.019.
- "The Benefits and Risks of Consuming Brewed Tea: Beware of Toxic Element Contamination".
- Karak T, Bhagat RM (2010). "Trace elements in tea leaves, made tea and tea infusion: A review". Food Research International (Review) 43 (9): 2234–2252. doi:10.1016/j.foodres.2010.08.010.
- Food and Drug Administration (24 February 2011). "Summary of Qualified Health Claims Subject to Enforcement Discretion". Retrieved 9 October 2014.
- National Cancer Institute (Reviewed 17 November 2010). "Tea and Cancer Prevention: Strengths and Limits of the Evidence". Retrieved 20 March 2012. Check date values in:
- Johnson R, Bryant S, Huntley AL (December 2012). "Green tea and green tea catechin extracts: an overview of the clinical evidence". Maturitas (Review) 73 (4): 280–7. doi:10.1016/j.maturitas.2012.08.008. PMID 22986087.
Green tea consumption does help reduce body weight and aid weight management as shown in short term RCTs (12 weeks) but not to a clinically relevant level.
- Wang W, Yang Y, Zhang W, Wu W (April 2014). "Association of tea consumption and the risk of oral cancer: a meta-analysis". Oral Oncol (Meta-Analysis) 50 (4): 276–81. doi:10.1016/j.oraloncology.2013.12.014. PMID 24389399.
- Wang Y, Yu X, Wu Y, Zhang D (November 2012). "Coffee and tea consumption and risk of lung cancer: a dose-response analysis of observational studies". Lung Cancer (Meta-Analysis) 78 (2): 169–70. doi:10.1016/j.lungcan.2012.08.009. PMID 22964413.
- Zheng J, Yang B, Huang T, Yu Y, Yang J, Li D (June 2011). "Green tea and black tea consumption and prostate cancer risk: an exploratory meta-analysis of observational studies". Nutr Cancer (Meta-Analysis) 63 (5): 663–72. doi:10.1080/01635581.2011.570895. PMID 21667398.
- Lin YW, Hu ZH, Wang X, Mao QQ, Qin J, Zheng XY, Xie LP (February 2014). "Tea consumption and prostate cancer: an updated meta-analysis". World J Surg Oncol (Meta-Analysis) 12: 38. doi:10.1186/1477-7819-12-38. PMC 3925323. PMID 24528523.
- Zheng JS, Yang J, Fu YQ, Huang T, Huang YJ, Li D (January 2013). "Effects of green tea, black tea, and coffee consumption on the risk of esophageal cancer: a systematic review and meta-analysis of observational studies". Nutr Cancer (Systematic Review and Meta-Analysis) 65 (1): 1–16. doi:10.1080/01635581.2013.741762. PMID 23368908.
- Shen L, Song LG, Ma H, Jin CN, Wang JA, Xiang MX (August 2012). "Tea consumption and risk of stroke: a dose-response meta-analysis of prospective studies". J Zhejiang Univ Sci B (Review) 13 (8): 652–62. doi:10.1631/jzus.B1201001. PMC 3411099. PMID 22843186.
- Larsson SC (January 2014). "Coffee, tea, and cocoa and risk of stroke". Stroke (Review) 45 (1): 309–14. doi:10.1161/STROKEAHA.113.003131. PMID 24326448.
- Hartley L, Flowers N, Holmes J, Clarke A, Stranges S, Hooper L, Rees K (June 2013). "Green and black tea for the primary prevention of cardiovascular disease" (PDF). Cochrane Database Syst Rev (Systematic Review and Meta-Analysis) 6: CD009934. doi:10.1002/14651858.CD009934.pub2. PMID 23780706.
- Liu G, Mi XN, Zheng XX, Xu YL, Lu J, Huang XH (October 2014). "Effects of tea intake on blood pressure: a meta-analysis of randomised controlled trials". Br J Nutr (Meta-Analysis) 112 (7): 1043–54. doi:10.1017/S0007114514001731. PMID 25137341.
- M. H. Wong, K. F. Fung and H. P. Carr (2003). "Aluminium and fluoride contents of tea, with emphasis on brick tea and their health implications". Toxicology Letters 137 (12): 111–120. doi:10.1016/S0378-4274(02)00385-5. PMID 12505437.
- Yi Lu, Wen-Fei Guo, and Xian-Qiang Yang (2004). "Fluoride Content in Tea and Its Relationship with Tea Quality". J. Agric. Food Chem. 52 (14): 4472–4476. doi:10.1021/jf0308354+S0021-8561(03)00835-5. PMID 15237954.
- K. F. Fung, Z. Q. Zhang1, J. W. C. Wong and M. H. Wong (1999). "Fluoride contents in tea and soil from tea plantations and the release of fluoride into tea liquor during infusion". Environmental Pollution 104 (2): 197–205. doi:10.1016/S0269-7491(98)00187-0.
- Lung SC, Cheng HW, Fu CB (2008). "Potential exposure and risk of fluoride intakes from tea drinks produced in Taiwan". J Expo Sci Environ Epidemiol 18 (2): 158–66. doi:10.1038/sj.jes.7500574. PMID 17410113.
- Malinowska E, Inkielewicz I, Czarnowski W, Szefer P (2008). "Assessment of fluoride concentration and daily intake by human from tea and herbal infusions". Food Chem. Toxicol. 46 (3): 1055–61. doi:10.1016/j.fct.2007.10.039. PMID 18078704.
- "Calls for FDA to introduce tea fluoride safety standard". Foodnavigator-usa.com. Retrieved 2011-08-04.
- Fung KF and MH Wong (2004) Application of different forms of calcium to tea soil to prevent aluminium and fluorine accumulation. J Sci Food Agric 84:1469–1477
- Fawell, J (2006). 92 4 156319 2. WHO. ISBN 92 4 156319 2.
- Jianyun Ruan and Ming H. Wong (2001). "Accumulation of Fluoride and Aluminium Related to Different Varieties of Tea Plant". Environmental Geochemistry and Health 23 (1): 53–63. doi:10.1023/A:1011082608631.
- (25 July 2013) Do fluoride levels in cheap tea pose a health risk? British National Health Service "Choices, Retrieved 26 July 2013
- Chen B, Shi HF, Wu SC (March 2014). "Tea consumption didn't modify the risk of fracture: a dose-response meta-analysis of observational studies". Diagn Pathol 9: 44. doi:10.1186/1746-1596-9-44. PMC 4017777. PMID 24588938.
- Michael Liebman and Shawnna Murphy (2007). "Low oxalate bioavailability from black tea". Nutrition Research 27 (5): 273–278h. doi:10.1016/j.nutres.2007.04.004.
- Bennett Alan Weinberg and Bonnie K. Bealer (2001). The World of Caffeine: The Science and Culture of the World's Most Popular Drug. Routledge. p. 228. ISBN 0-415-92722-6. Retrieved 2008-09-20.
- M. B. Hicks, Y-H. P. Hsieh, L. N. Bell, Tea preparation and its influence on methylxanthine concentration, Food Research International 29(3-4) 325-330 (1996)
- Graham H. N.; Green tea composition, consumption, and polyphenol chemistry; Preventive Medicine 21(3):334-50 (1992)
- "Environmental Pollution: Fluoride contents in tea and soil from tea plantations and the release of fluoride into tea liquor during infusion". ScienceDirect. Retrieved 2009-07-15.
- "Caffeine and Tea Information". Stash Tea. Retrieved 2009-07-15.
- BDA Supports Dehydration Awareness Week with some Handy Tips. British Dietetic Association press release, June 2011.
- Tea and Cancer Prevention - National Cancer Institute press release
- Green Tea (an overview from Drugs.com)